Volume control system for sound recording machines



Nov. 2, 1943. E. w. BEAL 2,333,031

VOLUME CONTROL SYSTEM FOR SOUND RECORDING MACHINES Filed May 14, 1942 JNVENTOR.

I Edward 14/, Bea] I v BY Q r Ala his Ai'hfs.

for limiting the Patented Nov. 2, 1943 VOLUME CONTROL SYSTEM FOR. SOUND RECORDING MACHINES Edward w. Beal, Wethersfleld, Conn.,' assi'gnor to The Gray Manufacturin Conn.,'a corporation oi' Application May 14, 1942, Serial No. 442,954

Claims.

This invention is concerned with improvements in sound recording and reproducing machines with special relation to a novel arrangement of volume limiting or control effective during recording.

The prime object of this invention is to provide in a sound recordingv machine of the type producing a permanent record on a disc or tape capable of use for reproduction immediately after recording, automatically operating means sound energy actuating the recording device so as to prevent overlapping of the individual sound records or tracks due to excess signal strength.

Another objector the invention is to provide in such a system a simple mechanism for cutting out the volume control or limiting circuit of this invention when desired.

A more specific object of thi invention is to provide in a sound recording and reproducing system a single manual control for conditioning the system for sound recording or reproduction.

With regard to the volume control feature, it is another object oi! this invention to provide an automatic volume control which eifects no control unless and until the strength of the signal energy being recorded rises to a, predetermined value. e

Other objects oi this invention will be clearly set forth in connection with the following description of one embodiment thereof.

This invention resides substantially in the combination, construction, arrangement and relative location of parts, as will be described in detail below.

In the drawing, the singl figure is a diagrammatic illustration of a preferred embodiment of the subject matter of this invention as applied to a sound recording machine of the dictation type.

Although the subject matter oi this invention, as will be apparent from the further description, is adaptable to abroader field of use, a description thereof will be given in connection with a sound recording machine adapted for dictation purposes where a permanent record is made upon a disc or tape which is capable of immediate reproduction. Modern machines of this type commonly employ a multi-stage vacuum tube amplifier energized by means of a power pack and easily connectable for recording or reproduction as desired.

In the case, for example, of recording sounds on a disc, the stylus of the recording head undergoes relative circumferential movement with reg Company, Hartford, Connecticut (Cl. I'm-100.4)

spect to a recording disc so that a record track or groove is formed on the disc as a continuous spiral path composed of closely adjacent convolutions; Frequently these convolutions are exceedingly close together being spaced at times not more than one thousandth of an inch with the result that in recording sounds of large volume the movement of the stylus of the recording head becomes sufliciently great so as to overlap into the adjacent groove previously made and into the area where the succeeding groove will be made. Naturally, if the recording stylus is thus caused to breakthrough into the adjacent groove areas, the eilect will be to introduce distortion into the sound records when reproduced which may be sufllciently disturbing to destroy the utility of the record.

In machines of this type, this condition has a strong tendency of arising'because of the fact that the overall sensitivity of the recording system must be made high in order that weak sounds may be recorded with certainty. .With a system of such overall sensitivity it becomes at once apparent that the volume of the sound being recorded does not have to rise very much before the condition of overlapping referred to above occurs. In accordance with this invention, an automatic volume control is provided of a specialized nature in that it play no part in the operation of th system until the signal strength or amplitudeof sounds to be recorded has risen to a predetermined value which can be readily controlled by design and adjustments to a desired value.

In the attached drawing, the multi-stage vacuum tube amplifier illustrated is but one example of many forms of such amplifiers available for use. For this reason, the various circuit adjuncts have not been numbered and will not be referred to except in so far as they have a bearing on the invention herein. The microphone l is connected to the input circuit of the first stage triode 2, the output circuit of which is coupled to the input circuit of the pentode 3 of the second stage which includes the tone control impedance 4. The output circuit of the pentode 3 in turn is coupled to the input circuit of the tetrode 5 in whose output circuit is connected the primary of the output transformer 6. One terminal of the secondary of this transformer is connected through a switch 1 to the grounded loud speaker 8. Switch 1 is controlled by a shaft 9 having an operating manual III at one end and a suitable support or carrier II at the other end by means of which the reproducing head I! and the recording head l3 may alternatively be applied to the record disc so as to be engaged by the styli of these heads respectively. A connection from the output circuit of the tetrode 5 is made through a coupling condenser It to a switch I5 in circuit with the recording head l3. As in the case of switch I, the movable contact of switch I5 is operated by the shaft 9. The power pack for this circuit comprises a full wave vacuum rectifier- I6 and associated filter network 111 in accordance with well known principles in this art. The connection 3|! from a point of suitable potential in the filter network is made to the output circuit of the tetrode 5. The reproducing head is connected by wire l9 through a rounded potentiometer 20 to the grid of a triode vacuum device l8. The cathode thereof is connected to the proper terminal of the filter network l1 through the usual circuit adjuncts. The anode potential for the triode is provided through the wire 2|, resistor 22 and switch 23 to the other terminal of the filter network. The movable contact oi switch 23 is likewise operated by the shaft 9. In the actual apparatus, as will be fully understood by those skilled in the art, the movable contacts of the switches I, I5 and 23 are operated by the shaft 9 but are insulated therefrom as required in order to properly isolate the various circuits associated therewith. The filter network terminal connected to the movable contact 23 may be connected to wire 24 during recording to supply the proper potentials for the anodes of the triode 2 and the pentode 3.

At 25 is diagrammatically illustrated a voltage divider network composed of resistors and capacitors in' which is included a vacuum tube diode 26, the cathode of which is coupled through a condenser 21 to the output circuit of the tetrode 5. The resistance 26" connecting the. cathode of diode 26 to ground is also part in the A. 0.. network as well as being a part of the D. C. delay microphone l are converted into signal currents which are rapidly amplified and elementally fed through the amplifier transformer 6 in a well known manner to the recording head i3, through the closed switch 23. With the record tablet in motion, these sounds will be recorded thereon. A control biasing voltage is applied to the cathode of the diode 26 through the connection including the condenser 21 from the output of the tetrode 5. As the volume of sound being recorded increases the negative bias on the cathode of the diode 26 likewise increases until it reaches a value where a current begins to fiow through the diode. Thereupon a current flows in the voltage divider network 25 causing the appearanc of a biasing control voltage on the control grid of the pentode 3. This control grid or electrode is biased in a sense or direction to change the gain of the pentode 3 which is proportional to the strength of the signal current in the output circuit of the tetrode and in turn proportional to the volum of the sound being imposed upon the microphone I. Thus above a particular signal strength, as determined by the initial fixed bias on the cathode of the diode 26, the volume control device comes into play to alter the gain of the amplifier in a bias network. The resistance 26' provides a D. C.

return circuit for the grid of tube 3. A connection is made from a point of suitable potential in the filter network l1 by means of wire 3| to the cathode of the diode 26. The resistance 3| in the wire 3| prevents shorting the condenser 21 to ground for A. C. potentials through the condenser in filter IT. The output end of the voltage divider network 25 is connected by means of a wire 28 to the negatively biased control grid of the pentode 3. through a suitable biasing resistor as shown. A return circuit connection shown from the voltage divider network 25 to the cathod circuit of the amplifier. A filter condenser is provided which may be short circuited by means of a switch 29 which whenclosed short circuits the voltage divider network and renders it inoperative.

In the operation of this circuit for recording, the manual I0 is employed to position the shaft 9 so that switch l5 closes on its contact, switch 1 opens on its contact, and switch 23 closes on its right hand contact. At the same time, the stylus of the recording head l3 engages the record disc. Assuming that the power pack is energized and the circuits to the cathodes of the various vacuum sense of decreasing it in order to prevent excess energization of-the recording head. It follows, therefore, that the record groove being made cannot overlap into adjacent record groove areas. As soon as th signal current strength falls below a predetermined value the volume control circuit again becomes inoperative. If atany time the volume control feature is undesired, it is only necessary to close switch 29 to render the volume control device inoperative. In passing, at this point, it may be noted that the potentiometer 4 in the input circuit of the tetrode '3 is atone control device which forms no part of this invention.

In order to reproduce a sound record, the shaft 9 is operated by the manual ID to the position shown in the drawing whereupon switch I5 opens, switch closes and switch l3 closes on its 7 left hand contact. The same movement of shaft 3 disengages the recording head from the record and causes th stylus of the reproducing head to ngage the record. The opening of switch |5 disconnects the recording head |3 from circuit. The closing of switch I connects the loud speaker 8 to the secondary of the transformer 6. Switch 23 in the position shown connects the power pack H to the anode of the triode l8 through resistor 22 and wire 2|. Wire 2|, it will be recalled, is connected to the input circuit of the tetrode 5. The output circuit of the tetrode is supplied with operating potential through the wire 3|. Electrical currents generated by the reproducing head devices are energized the apparatus isin condipower pack is at such a negative value that the diode 26- is blocked. Sounds spoken into the I2 are impressed upon the control electrode of the triode l8 through the wire I9 and their strength may be controlled as desired by the potentiometer 20. The output circuit of the triode I8 is connected totheinput circuit of the tetrode 5. The amplified currents produced by the tetrode are supplied to the loud speaker through the transformer 6 and switch 1. At this time, 'the automatic volume control devic is inoperative because control of the. amplitude of output energy may readily be accomplished through the potentiometer 20.

and 23 are open.

From the above description, it will .be apparent to those skilled in the art that the subject matter of this invention is capable of adaptation to many uses and in different physical forms and I do not, therefore, desire to be strictly limited to the details of the disclosure given for the purpose of illustrating these principles but rather to the true legal scope of the claims granted me.

What is claimed is:

1. In a machine for making a sound record, the combination including a recording head, a microphone, a multi-stage vacuum tube amplifier connecting said microphone with said recording head, a voltage divider network including a diode vacuum device, a connection from the output of said amplifier to the cathode of said diode, a connection from said voltage divider network to the input circuit of one of the stages of said amplifier and means for biasing said diode so that it is in a blocked condition until the signal currents in said output circuit exceed a predetermined value whereby the gain of said amplifier is decreased when the volume of sound impressed upon the microphone exceeds a predetermined value.

2. In a machine for making a sound record, the combination including a recording head, a microphone, a multi-stage vacuum tube amplifier connecting said microphone with said recording head, a voltage divider network including a diode vacuum device, a connection from the output of said amplifier to the cathode of said diode, a connection from said voltage divider network to the input circuit of one of the stages of said amplifier, means for biasing said diode so that it is in a blocked condition until the signal curt rents in said output circuit exceed a predetermined value whereby the gain of said amplifier is decreased when the volume of sound impressed upon the microphone exceeds a predetermined value and means for rendering said voltage divider network inoperative.

3. In a machine for making a sound record, the combination including an electrically operated recording head, a microphone, a multistage vacuum tube amplifier coupling said microphone with said recording head, a power pack for supplying operating potentials to said amplifier, means controlled from the output of said amplifier for decreasing the gain of said amplifier when the output signals of the amplifier exceed a predetermined value and a connection from said power pack to said last means for biasing it to predetermine said value.

4. In a machine for making a sound record, the combination including an electrically operated recording head, a microphone, a multistage vacuum tube amplifier coupling said microphone with said recording head, a power pack for supplying operating potentials to said amplifier, a biasing potential generator including a diode vacuum device, a connection from the output circuit of the amplifier to the cathode of said diode, a connection from the anode of said diode to the control grid of the vacuum device of one of the stages of the amplifier and means for supplying a predetermined biasing voltage to the cathode of said diode whereby the gain of said amplifier is decreased when the signal strength in its output circuit exceeds a predetermined value.

5. In a machine for making a sound record, the combination including an electrically operated recording head, a microphone, a multistage vacuum tube amplifier coupling said microphone with said recording head, a power pack for supplying operating potentials to said amplifier, a biasing potential generator including a biased diode vacuum device, a connection from said generator to the input circuit of one of the stages of said amplifier and means controlled by the signal energy in the output of the amplifier for varying the bias of said diode whereby the gain of said amplifier is reduced when the volume of sounds being recorded exceeds a predetermined value.

EDWARD W. IBEAL. 

